Devices for adding and dropping wavelength coded signals (light of a specific wavelength or wavelengths) are known in the art. Such devices employ optical fibers which are utilized predominantly in telecommunication systems in addition to local area networks, computer networks and the like. The optical fibers are capable of carrying large amounts of information and it is the purpose of devices of the present invention to extract a selected amount of information from the fiber by segregating the information carried on different wavelength channels.
Devices of this type are comprised of a variety of components which together provide the desired segregation of wavelength coded signals. Integrated optical couplers and especially directional couplers have been developed to accomplish evanescent directional coupling. Optical signals are coupled from one planar waveguide to another. The signals in the second planar waveguide propagate in the same direction in which the signals travel in the first planar waveguide.
Diffraction gratings (e.g. Bragg gratings) are used to isolate a narrow band of wavelengths. Such grating reflectors have made it possible to construct a device for use in adding or dropping a light signal at a predetermined centered wavelength to or from a fiber optic transmission system without disturbing other signals at other wavelengths.
Bragg grating systems for use in optical coupler devices are disclosed in Elias Snitzer, U.S. Pat. No. 5,574,807, incorporated herein by reference. The '807 Patent discloses a device for use in adding or dropping light signals at predetermined center wavelengths to or from a wavelength division multiplex fiber optic transmission system which carries signals at other wavelengths. A twin core fiber which is comprised of two, substantially identical, single mode fibers which form a coupling region are fabricated to provide substantially complete evanescent field coupling of light from one core to the other in a predetermined wavelength band. The twin core fiber also includes a Bragg grating system which is substantially perpendicular to the axis of the twin core fiber.
Charles H. Henry et al., U.S. Pat. No. 5,636,309 discloses Mach-Zehnder (MZ) type devices which employ two planar optical waveguides (e.g. fibers), with each of the waveguides comprising a core and a cladding. The waveguides include opposed arms that extend between two coupling regions. In the coupling regions the cores of the respective waveguides are disposed relatively close to each other to facilitate evanescent field coupling. The spaced apart arms are disposed such that the maximum spacing between the arms is small enough to enable simultaneous formation of the refractive index Bragg gratings by simultaneous exposure of both cores to refractive index-changing radiation. U.S. Pat. No. 5,636,309 is incorporated herein by reference.
The devices disclosed in the prior art such as described above suffer from a number of disadvantages. One such disadvantage is that they are difficult to fabricate. A second disadvantage is that it is difficult to align Bragg gratings across the arms of the optical waveguides in the grating region. A third disadvantage is that there is a significant loss of light owing to the length of the waveguides.
It would therefore be a significant advance in the art of optical waveguide devices to provide such devices with greater control over the passage of light through the device and control over the adding and/or dropping of light signals at a predetermined wavelength or wavelengths.